1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more specifically to a method for forming a hole filling plug.
2. Description of Related Art
As a technology for filling a fine contact hole or via-hole, a formation of a tungsten (W) plug by a blanket WCVD (tungsten chemical vapor deposition), has been reduced into practice, which is excellent in step coverage and low in resistance. This blanket WCVD method is a method of charging a W layer into a via-hole formed in an interlayer insulator film so as to form a plug.
Referring to FIGS. 1A to 1C, there are shown diagrammatic sectional views illustrating a process for forming a via-hole filling W plug in accordance with a conventional blanket WCVD process. As shown in FIG. 1A, an interlayer insulator film 33 is formed on a first TiN (titanium nitride) film 32 formed on a lower level wiring Al conductor 31, and a via-hole 33A is formed through the interlayer insulator film 33 by a conventional photolithography and an anisotropic dry etching, so that a portion of the first TiN film 32 is exposed at a bottom of the via-hole 33A.
Then, as shown in FIG. 1B, a second TiN film 34 is deposited by a sputtering so as to cover a whole surface including the bottom and a side wall of the via-hole 33A. Furthermore, a W (tungsten) layer 36 is deposited in the via-hole 33A and the surface of the interlayer insulator film 33, and succeedingly, the deposited W layer 36 is etched back so that the W layer 36 excluding the W layer filled in the via-hole 33A is removed. Thus, as shown in FIG. 1C, a W plug consisting of the W layer 36 is formed in the via-hole 33A. This W plug consisting of the W layer 36 is used to interconnect the lower level wiring Al conductor 31 and a not-shown upper level wiring Al conductor which are insulated from each other by the interlayer insulator film 33.
In the above mentioned conventional process, the second TiN film 24 is formed as an adhesion layer, ordinarily by sputtering. In this sputtering process, a growth speed of the TiN film 34 is higher at an opening edge of the via-hole than at the other regions. As a result, it has been confirmed by experiments that, as shown in FIGS. 1B and 1C, the TiN film 34 has an overhanging portion 34A around the opening edge of the via-hole 33A. If the W layer is deposited by the blanket WCVD process in a condition that the TiN film 34 has the overhanging portion 34A, the deposited W layer 36 reflects the shape of the overhanging portion 34A of the TiN film 34, with the result that a void 39 occurs in the W layer 36 charged in the via-hole 33A. The cause for this is considered that since the overhanging portion 34A of the TiN film 34 projects inwardly from around the opening edge of the via-hole 33A, the overhanging portion 34A of the TiN film 34 becomes a growth core when the W layer 36 is deposited, and therefore, the W layer 36 starts to grow from around the opening edge of the via-hole 33A, so that the opening of the via-hole 33A is closed before the whole of the via-hole is completely charged or filled with the W layer 36.
In order to prevent the overhanging of the sputter-deposited TiN film from the opening edge of the via-hole 33A, it is considered, as shown in FIG. 2, to form the second TiN film 34 on the interlayer insulator film 33 by sputtering, then to form a via-hole 33A through the second TiN film 34 and the interlayer insulator film 33 by a conventional photolithography and an anisotropic dry etching, and thereafter to perform the blanket WCVD process. In this process, however, since an inner edge 34B of the TiN film 34 is exposed around the opening edge of the via-hole 33A, the inner edge 34B of the TiN film 34 becomes a growth core when the W layer 36 is deposited. As a result, a void 39 occurs in the W layer 36 filled in the via-hole 33A, similarly to the example shown in FIG. 1C. This has been also confirmed by experiment.
If the void 39 occurs in the W layer 36 filled in the via-hole 33A as shown in FIGS. 1C and 2, wiring defects such as disconnects are apt to occur easily. In order to prevent this problem, it is necessary to prevent generation of the void. For preventing generation of the void, it it required to suppress the growth of the W layer around the opening edge of the via-hole 33A, so that the W layer is uniformly grown from the bottom of the via-hole to fill the whole of the via-hole.